Hydrostatic machine comprising a cam ring with adjacent bearings of same outer diameter, and manufacturing method

ABSTRACT

A hydrostatic machine comprising a stator and a rotor. The stator comprises an inner cylindrical surface with a constant diameter and a cam ring comprising a cam track on the inner circumference thereof, the outer circumference of the cam ring being mounted in the aforementioned inner cylindrical surface. The hydrostatic machine also comprises two bearings that allow the rotation of the rotor in relation to the stator and that are mounted on the inner cylindrical surface of the stator, axially on either side of the cam ring, the cam ring and the two bearings having the same outer diameter.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.17/257,512, filed Dec. 31, 2020, which is a national phase entry under35 U.S.C. § 371 of International Patent Application PCT/FR2019/051648,filed Jul. 3, 2019, designating the United States of America andpublished as International Patent Publication WO 2020/008145 A1 on Jan.9, 2020, which claims the benefit under Article 8 of the PatentCooperation Treaty to French Patent Application Serial No. 1856142,filed Jul. 3, 2018. The disclosure of the aforementioned U.S. patentapplication Ser. No. 17/257,512 is hereby incorporated in its entiretyherein by this reference.

TECHNICAL FIELD

The disclosure relates to the field of mechanics and of hydraulics andparticularly relates to a hydrostatic machine.

BACKGROUND

Hydrostatic machines generally have one end connected to a structure andanother end connected to a rotating drive element such as a wheel, apropeller or any transmission device.

Such a hydrostatic machine may be used as a hydraulic motor. It is thensupplied with a pressurized hydraulic fluid and in response drives therotating drive element.

The hydrostatic machine may also be employed as a hydraulic pump. Itthen receives a torque transmitted by the rotating drive element and inresponse compresses the hydraulic fluid.

Patent application FR3030381 describes a hydraulic motor comprising:

-   -   a stator equipped with fixings for a structure and comprising a        circumferential cam track;    -   a rotor provided with fixings for a rotating drive element and        comprising circumferentially distributed pistons suitable for        interacting with the cam track; and    -   a hydraulic distributor suitable for selectively supplying the        pistons with hydraulic fluid so that the interaction of the        pistons with the cam track corresponds to a relative rotation of        the rotor with respect to the stator.

BRIEF SUMMARY

This disclosure describes improvements to the aforementioned type ofmachine from the point of view of compactness, robustness, and of themanufacturing process.

To this end, embodiments of the disclosure relate to a hydrostaticmachine comprising:

-   -   a stator equipped with fixings for a structure and comprising a        circumferential cam track;    -   a rotor provided with fixings for a rotating drive element and        comprising circumferentially distributed pistons suitable for        interacting with the cam track;    -   and    -   a hydraulic distributor suitable for selectively supplying the        pistons with hydraulic fluid so that the interaction of the        pistons with the cam track corresponds to a relative rotation of        the rotor with respect to the stator.

The hydrostatic machine has the following features:

-   -   the stator comprises an internal cylindrical surface (e.g., an        inner cylindrical surface) of constant diameter and a cam ring        that comprises, on its internal circumference, the cam track,        and that is mounted, via its external circumference, in the        internal cylindrical surface; and    -   it comprises two bearings that allow the rotation of the rotor        with respect to the stator, and that are mounted on the internal        cylindrical surface of the stator, axially on either side of the        cam ring, the cam ring and the two bearings having the same        outside diameter.

Another subject to which embodiments of the disclosure relate is aprocess for manufacturing a hydrostatic machine, comprising thefollowing acts:

-   -   machining a tube of bearing steel to produce a cam ring having a        cam track;    -   mounting the cam ring and two bearings on a main body of a        rotor, the cam ring being clamped between the two bearings, to        form a sub-assembly; and    -   axially inserting the sub-assembly into a stator that comprises        an internal cylindrical surface of constant diameter, the        bearings and the cam ring becoming mounted in the internal        cylindrical surface.

Such a hydrostatic machine has an increased compactness, this beingparticularly advantageous when the hydrostatic machine is intended to befitted in the wheel of a vehicle with a view to powering the latter. Inthe latter case, the more compact the hydrostatic machine, the betterable it is to fit into the wheel rim of the vehicle.

On the internal cylindrical surface of the stator are mounted both thepivot elements linking with the rotor (the bearings) and the rotatingdrive elements of the rotor (the cam ring). No complex device orgeometric arrangement is required to hold these elements axially, thecylindrical surface being of constant diameter.

Optionally, a shoulder may be provided on the stator at the end of thisinternal cylindrical surface and is enough to allow both the twobearings and the cam ring to be put in position in the stator.

Since the internal cylindrical surface is devoid of element(s) forpositioning the bearings, the cam ring may then combine its function asa carrier of the cam track with a function as a positioning spacerbetween the two bearings, this contributing to the axial compactness ofthe machine. The axial compactness of the machine may be furtherimproved by providing, on the cam ring, an annular abutment that enablesplacement against the external races of the bearings, as close to thebearing cages as possible, without however hampering the rotation of thelatter.

According to one preferred feature, the stator comprises a tubularcasing, this promoting radial compactness. The internal cylindricalsurface of the rotor is then supported by a wall that is thin comparedto the other dimensions of the machine.

Specifically, since the cam track is borne by a part (the cam ring) thatis separate from the rest of the stator, the function of interactingwith the pistons is decoupled from the function of structurally holdingelements linked with the stator.

The cam track must have high hardness and high resistance to the wearcaused by the pistons rolling over the cam track. These properties aregenerally provided by fragile materials, such as hardened steel. The camring is, therefore, advantageously made of such a material because itsonly dynamic function is to interact with the pistons.

The tubular casing, for its part, performs the function of structurallyholding the elements that the stator contains and must, in contrast,have a certain ductility so as not to break or crack under the effect ofshocks or any deformation during operation of the machine, which issupplied with a hydraulic fluid under high pressure. These propertiesare given to the stator via the choice of a ductile material, and of athin thickness for the wall of the stator, which may thus bend. Bendingis promoted, at least in the segment of the stator neighboring theinternal cylindrical surface, by the fact that the stator does notrequire, in this segment, any functional areas of larger thickness orgeometric shapes to position elements or to stiffen the stator.

In the prior art, the cam track is generally machined in the body of thestator and a surface-hardening heat treatment is provided in addition.Such a stator is difficult and expensive to produce.

According to one preferred feature of the disclosure, the cam ring maybe made from a steel of the grade referred to as “bearing steel,” or“carbon steel,” which has a high proportion of carbon, a high resistanceto wear and fatigue, but which is, however, sensitive to shocks. Theweakness of the cam ring with respect to shocks is compensated for bythe fact that it is mounted in the tubular casing of the rotor, which isductile.

Embodiments of the disclosure thus allow advantage to be taken of thehigh performance of a material that is resistant to contact pressure andto fatigue to produce the cam track without suffering from the drawbacksnormally associated with this type of material.

This assembly moreover allows the process for manufacturing thehydrostatic machine to be considerably simplified.

Specifically, during the assembly of such a machine, a main body of therotor may be equipped beforehand with the two bearings and the cam ring,the two bearings framing on either side the cam ring while holding itaxially. This sub-assembly consisting of the main body of the rotor, ofthe bearings, and of the cam ring may then be, in a single operation,mounted inside the stator so that the two bearings and the cam ring areslid along the internal cylindrical surface. The assembly operationsare, therefore, considerably simplified.

The number of operations required to produce the stator is alsodecreased because of the presence of the tubular stator casing, whichmay be produced from a steel tube requiring few or no machiningoperations. The cam ring may be produced by machining a tube of bearingsteel, which is inexpensive because it is produced in large volumes forthe manufacture of bearings, and which has excellent properties inrespect of hardness and resistance to contact fatigue.

The production of such a hydrostatic machine is, therefore, faster andless expensive.

The hydrostatic machine may in addition comprise the followingadditional features, alone or in combination:

-   -   the hydrostatic machine comprises a tubular casing, the internal        cylindrical surface being defined by the tubular casing;    -   the cam ring has a ductility lower than that of the tubular        casing;    -   the cam ring is made of bearing steel and the tubular casing is        made of non-alloy steel or austenitic stainless steel;    -   the material of the cam ring and that of the two bearings is the        same bearing steel;    -   the cam ring is fitted tightly in the tubular casing;    -   the machine comprises anti-rotation fixings for coupling the cam        ring and the stator;    -   the machine comprises: a clamping ring placed axially against        one of the bearings and against the rotor; and a lip seal placed        between the clamping ring and the stator;    -   the clamping ring is fastened to the rotor by a hub screwed into        the rotor and bearing the fixings (e.g., fasteners) for a        rotating drive element; and/or    -   the fixings (e.g., fasteners) for a rotating drive element        consist of screws the heads of which are clamped by the hub.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred example of an embodiment of the disclosure will now bedescribed with reference to the accompanying drawings, in which:

FIG. 1 shows a hydrostatic machine according to the disclosure, seenfrom the side;

FIG. 2 is a perspective view showing the machine of FIG. 1 , from therotor side;

FIG. 3 is a face-on cross-sectional view of section AA of FIG. 1 ;

FIG. 4 is a side cross-sectional view of section BB of FIG. 3 ;

FIG. 5 is a schematic illustrating a process for manufacturing themachine of FIGS. 1 to 4 ;

FIG. 6 shows, in cross section, a sub-assembly intended for producingthe machine of FIGS. 1 to 4 ;

FIG. 7 shows in perspective the tubular casing of the machine of FIG. 1; and

FIG. 8 shows in perspective the cam ring of the machine of FIG. 1 .

DETAILED DESCRIPTION

FIGS. 1 and 2 show a hydrostatic machine 1 according to embodiments ofthe disclosure, seen in profile and in perspective from the rotor side,respectively.

The hydrostatic machine 1 has a generally cylindrical shape andcomprises a stator 2 and a rotor 3. A relative rotational movement isallowed between the stator 2 and the rotor 3, around an axis X. Thegenerally cylindrical shape is adapted to the internal make-up of themachine and allows it to be mounted, at least partially, in acylindrical element relatively to the rotating drive element, in the rimof a wheel for example.

In the present example, the hydrostatic machine 1 is intended to befastened to a structure consisting of the chassis of a vehicle (notshown). A wheel (not shown) is mounted on the rotor of the machine sothat the vehicle may be propelled by the rotation of the wheel.

The hydrostatic machine 1 comprises, on the structure side (on the leftin FIG. 1 ) means (e.g., threaded bores 15) for fastening to thestructure and means (e.g., hydraulic connectors 16) for supplyinghydraulic fluid with a view to supplying power to the hydrostaticmachine 1.

On the wheel side (on the right side in FIG. 1 ), the hydrostaticmachine 1 comprises a wheel hub 4 that forms part of the rotor 3. Thiswheel hub 4 comprises (e.g., bears) fastening means (e.g., fasteners)for a rotating drive element. In the present example, the rotating driveelement is a vehicle wheel (not shown) and the fastening means are studs5 for fastening the vehicle wheel. Accordingly, the hydrostatic machine1 may include a hub (e.g., wheel hub 4) bearing (e.g., supporting)fasteners (e.g., studs 5) for a rotating drive element.

The hydrostatic machine 1, being thus fastened to a structure by itsstator 2, and being attached to a vehicle wheel by its rotor 3, mayoperate in two modes:

-   -   a motor mode in which the energy of the pressurized fluid is        converted into mechanical energy and causes the rotation of the        wheel and, therefore, the movement of the vehicle; and    -   a generator mode in which the wheel is driven to rotate by the        environment (for example, when the vehicle is on a downward        slope) and itself drives the rotor 3 to rotate to place the        hydraulic fluid under pressure.

FIG. 2 shows the fastening holes of the wheel hub 4 on the rest of therotor 3.

FIG. 3 is a cross-sectional view of section A-A of FIG. 1 andillustrates the operating principle of the hydrostatic machine 1.

The portion of the rotor 3 that may be seen in FIG. 3 is its main body31. It is a circular part in which are drilled eight radial cylinders 6,which are distributed circumferentially around the main body 31 of therotor 3.

An orifice 7 for supplying hydraulic fluid opens into each of thesecylinders 6.

A piston 8 is inserted into each cylinder 6 so that pressurization ofthe hydraulic fluid via the supply orifice 7 causes the piston 8 to exitradially outwards and, conversely, the movement of the piston 8 when itis forced radially inwards causes hydraulic fluid to exit via the supplyorifice 7 (to simplify the figure, only three pistons 8 have been shownin FIG. 3 ).

Each piston 8 is equipped with a roller 9 that is movably mounted on thepiston 8 with respect to an axis parallel to the axis X.

Moreover, two elements of the stator 2 have been shown in FIG. 3 : atubular casing 10 and a cam ring 11.

The cam ring 11 is mounted in the tubular casing 10 and these elementsare secured together. Anti-rotation fixings allow the cam ring 11 andthe tubular casing to be secured together so as to rotate as one. In thepresent example, the anti-rotation fixings comprise holes 12 distributedaround the circumference of the tubular casing 10, and correspondingholes 13 in the cam ring 11, as well as screws (not shown) to ensure thefastening. The cam ring 11 comprises on its internal circumference a camtrack 14 to which a succession of recesses and bumps give a wavy shape.During the operation of the hydrostatic machine 1, the rollers 9 of thepistons 8 roll over the cam track 14.

The cam ring 11 is made of bearing steel, 100Cr6 steel for example. Thecam ring 11 is advantageously fitted tightly in the tubular casing 10.The tubular casing 10 is made of a more ductile material than the camring 11. Clamping the cam ring 11 in the tubular casing 10 allows thecam ring 11 to be kept in compression in the tubular casing 10, thiscontributing to preventing the appearance of fatigue cracks in the camring 11.

In a known way, the pistons 8 are selectively supplied with pressurizedfluid depending on their angular position with respect to the cam track14 so that the pressure of the fluid is converted into rotation of thecam track 14 and, therefore, of the rotor 3.

FIG. 4 is a cross-sectional view of section B-B of the hydrostaticmachine 1 of FIG. 3 .

The stator 2 is formed from two portions: a base 18 and a tubular casing10. FIG. 4 shows, from the structure side, the means for fastening thehydrostatic machine 1 to the structure of the vehicle. In the presentexample, these are threaded bores 15 that are regularly distributedaround the circumference of the tubular casing 10 and that allow it tobe fastened by screws to the structure (not shown).

The stator 2 also comprises, on the structure side, hydraulic connectors16 that are intended to connect the ducts of the hydraulic circuit ofthe vehicle with a view to supplying hydraulic fluid to the hydrostaticmachine 1.

These hydraulic connectors 16 are arranged on the base 18 and areconnected, by internal channels of the stator 2, to a hydraulicdistributor 17. The hydraulic distributor 17 is itself equipped withinternal ducts that allow hydraulic fluid to be selectively supplied tothe pistons 8.

The operation of the hydraulic distributor 17, and more generally of theselective supply of the pistons 8 with hydraulic fluid, takes place inaccordance with what is known in this field. This operation, which will,therefore, not be described in more detail here, allows pressurizedhydraulic fluid to be delivered to certain pistons 8, via their supplyorifice 7, and allows the hydraulic fluid to exit from certain otherpistons 8, via their supply orifice 7.

The tubular casing 10 is a tube fitted onto the base 18. In the presentexample, it is a question of a press fit that allows the base 18 to besecurely fastened to the tubular casing 10 without any other additionalfasteners. As a variant, screws or any other fastening means may beprovided to consolidate the assembly.

Where it is press fitted, the tubular casing 10 comprises a reinforcingshoulder 19. The threaded bores 15 for fastening the hydrostatic machine1 to the structure are here produced in this reinforcing shoulder 19.

From the reinforcing shoulder 19, the tubular casing 10 extends, in thedirection of the wheel side, such as to form a first shoulder 20, aninternal cylindrical surface 21 (also referred to herein as an “inner”cylindrical surface) of constant diameter, and a second shoulder 22,these elements being arranged in tiers, i.e., the inside diametersdefined by the first shoulder 20, the internal cylindrical surface 21,and the second shoulder 22, respectively, increase in the direction ofthe wheel side.

Apart from the reinforcing shoulder 19, the function of which is notrelated to interaction with the rotor 3 but only to fastening andassembly of the stator 2, the tubular casing 10, therefore, has threeinside diameters the largest of which is located on the wheel side.

On the internal cylindrical surface 21 are mounted:

-   -   a first bearing 23, the external race of which is placed axially        against the first shoulder 20;    -   the cam ring 11, which is axially positioned by its fixings        (e.g., corresponding holes 13); and    -   a second bearing 24, the external race of which is axially        placed against a clamping ring 29.

The cam ring 11 comprises an annular abutment 41 for mounting it in thetubular casing 10. The abutment 41 is clamped between the external racesof the two bearings 23, 24.

The bearings are here made of bearing steel, 100Cr6 steel for example.The external races of the bearings 23, 24 and the cam ring 11 are,therefore, made of the same, preferably through-hardened, material(100Cr6 steel).

As regards the rotor 3, the cross section of FIG. 4 allows the profileof two cylinders 6 and of their respective supply orifice 7 to be seen.

The rotor 3 is mounted so as to be able to rotate inside the tubularcasing 10 by virtue of the bearings 23, 24, which interact with the mainbody 31 of the rotor 3. The internal race of the first bearing 23 ismounted on the rotor 3 so that it abuts axially against a shoulder 25 ofthe rotor 3, this shoulder 25 being located on the structure side.

The second bearing 24 is mounted on the rotor 3, on the wheel side, sothat the two bearings 23, 24 lie on either side of each cylinder 6.

The end of the main body 31, which is on the wheel side, has a radialface 26 that coincides with the rim of the internal race of the secondbearing 24. The dimensions of the main body 31, of the bearings 23, 24,and of the cam ring 11, are chosen so that the succession of dimensionsbetween the shoulder 25 and the rim of the internal race of the secondbearing 24 leads to an alignment, in the same plane, of the radial face26 and of the rim of the internal race of the second bearing 24. Thus,an axial end of the internal race of the second bearing 24 lies in thesame plane as the radial face 26 of the main body 31.

The rotor 3 in addition comprises a clamping ring 29 that abuts axiallyboth against the internal race of the second bearing 24 and against theradial face 26.

As a variant, the aforementioned succession of dimensions causes the rimof the internal race of the second bearing 24 to be axially slightlybeyond the radial face 26, so that fastening the clamping ring 29pre-stresses the bearings 23, 24.

The rotor 3 also comprises the wheel hub 4, which is fastened againstthe radial face 26 of the main body 31 by the screws 32. The wheel hub 4has a shoulder 33, the axial dimension of which is equal to the axialdimension of the clamping ring 29.

Thus, when the wheel hub 4 is screwed against the main body 31, theclamping ring 29 is pressed both against the main body 31 and againstthe internal race of the second bearing 24, and held in this position.Accordingly, the clamping ring 29 may be fastened to the rotor 3 by ahub (e.g., wheel hub 4) screwed into the rotor 3. The hub (e.g., wheelhub 4) may bear (e.g., support) fasteners (e.g., studs 5) for a rotatingdrive element (e.g., a wheel), as described further below.

The rotor in addition comprises an O-ring 34 placed in a groove of themain body 31 and interposed between the latter and the clamping ring 29,in order to ensure seal tightness between these two elements.

Furthermore, a lip seal 35 is interposed between the clamping ring 29and the tubular casing 10. The lip seal 35 is placed in axial abutmentagainst the second shoulder 22.

The O-ring 34 and the lip seal 35 together form an outwardly seal-tightbarrier that confines, within the tubular casing 10, any hydraulic fluidthat may be found therein.

As a variant, while remaining clear of the bearing cage of the secondbearing 24, the lip seal 35 may be placed directly in abutment with thesecond bearing 24.

The wheel hub 4 comprises, as also shown in FIG. 4 , threaded holes 36for mounting the studs 5. Accordingly, a hub (e.g., wheel hub 4) maybear (e.g., support) fasteners (e.g., studs 5) for a rotating driveelement (e.g., wheel). In the present example, the studs 5 consist ofscrews having a head 38 that is, for example, a hexagonal socket head.Each threaded hole 36 is associated with a counterbore 37, the axialdimension of which is equal to the height of the corresponding head 38.

The head 38 of the screws forming the studs 5 is, therefore, blocked inboth axial directions: by the counterbore 37 on the right side (withreference to FIG. 4 ) and by the clamping ring 29 on the left side (theright and left sides are indicated with reference to FIG. 4 ).Accordingly, the fasteners (e.g., studs 5) for the rotating driveelement (e.g., wheel) may comprise screws with heads (e.g., heads 38)clamped by the hub (e.g., wheel hub 4). The height of the head 38 andthe axial dimension of the counterbore 37 are, therefore, chosen so thatthe studs 5 are unable to become loose in normal operation.

A dust-proof ring seal 39 may in addition be provided between the wheelhub 4 and the tubular casing 10. The dust-proof ring seal 39 comprises agroove 42 equipped with an axial stop. Thus, if the pressure in thecasing were to push the lip seal 35 outwards, contact with thedust-proof ring seal 39 and this axial stop will prevent anydislodgement of the seals 35, 39.

The threaded bores 15 that allow the hydrostatic machine 1 to befastened to a chassis are produced in the tubular casing 10 so that theforces are transmitted via a short mechanical path between the rotor andthe chassis, this path passing only through the bearings 23, 24 and thetubular casing 10.

The process of manufacturing the hydrostatic machine 1 will now bedescribed with reference to FIG. 5 , which schematically shows the mainsteps of the manufacturing process.

The base 18, the tubular casing 10 and the hydraulic distributor 17 areproduced in steps E1, E2 and E3, respectively. The base 18 and thehydraulic distributor 17 are produced by any conventional mechanicalmeans of manufacture, by molding and machining of the functional partsfor example. The tubular casing 10 is advantageously produced from atube of rolled steel of E470 grade (according to European steel gradedesignation system E10027), which has the advantage of being inexpensiveand of having a ductility sufficient for the job of the tubular casing10. The tubular casing 10 is thus advantageously made of a weldablesteel in order, optionally, to be able to weld therein any externalfixings required to mount the hydrostatic machine 1.

The thickness of this tube of E470 steel is equal to the intendedthickness of the reinforcing shoulder 19, the internal surface of thistube then being machined to form the first shoulder 20, the internalcylindrical surface 21 and the second shoulder 22. The holes 12 forfastening the cam ring 11 are lastly drilled in the tubular casing 10.The tubular casing 10 produced in step E2 is shown in FIG. 7 .

In step E6, the base 18 and the tubular casing 10 are assembled by pressfitting, then the hydraulic distributor 17 is placed on the base 18.

In parallel with the steps described above, the main body 31 and the camring 11 are manufactured in steps E4 and E5, respectively. The main body31 is also produced by any conventional mechanical means. The cam ring11 is advantageously produced from a tube of bearing steel, 100Cr6 steelfor example (according to European steel grade designation systemE10027), the outside diameter of which is substantially equal to thediameter of the internal cylindrical surface 21 of the tubular casing10, depending on how tightly it is desired for the cam ring 11 to fit inthe tubular casing 10. To produce the cam ring 11, a slice of such atube of bearing steel, of a dimension equal to the intended axialdimension of the base (e.g., abutment 41) of the cam ring 11, is firstcut. An annulus is thus obtained, and the internal surface of thisannulus is then machined with a digital milling machine to obtain thecam path (e.g., cam track 14) shown in FIG. 3 .

Lateral recesses are then machined in the cam ring 11 to form theabutment 41 of the cam ring 11. The abutment 41 is intended to beclamped between the external races of the two bearings 23, 24, and thelateral recesses allow the passage of the bearing cages, which projectaxially with respect to the internal race. Since the abutment 41 makescontact with the external races of the bearings 23, 24, the two sidefaces of the abutment 41 need to have a good planarity. The abutment 41is, therefore, ground after the lateral recesses have been machined(which also allows less material to be ground).

The corresponding holes 13 intended for fastening are then produced inthe tubular casing around the entire perimeter of the cam ring 11. Thecam ring 11 that results from operation E5 is shown in FIG. 8 .

In a step E7, the main body 31, the cam ring 11, and the two bearings23, 24 are assembled to obtain the sub-assembly shown in FIG. 6 . Thefirst bearing 23 is firstly mounted around the main body 31 until itabuts against the shoulder 25. The internal race of the first bearing 23(and also the internal race of the second bearing 24) may be assembledso as to slightly clamp the main body 31.

The cam ring 11 is then mounted around the main body 31 so as to abutagainst the first bearing 23. More precisely, the abutment 41 makescontact with the external race of the first bearing 23. The cam ring 11has, in this position, no radial support for its internal surface (thecam track 14) and must, therefore, be positioned so that its externalsurface is aligned with the external surface of the first bearing 23.

The second bearing 24 is then in turn mounted around the main body 31until its external race abuts against the base (e.g., the abutment 41)of the cam ring 11.

As described above, the dimensions of these various elements are chosenso that, once mounting of the sub-assembly has ended, the rim of theinternal race of the second bearing 24 coincides with the radial face26. The cam ring 11 is moreover positioned axially by the bearings 23,24.

In a step E8, the sub-assembly of FIG. 6 is inserted in a singleoperation into the tubular casing 10 (FIG. 7 ) until the external raceof the first bearing 23 abuts against the first shoulder 20 of thetubular casing 10. The cam ring 11 is advantageously fitted tightly inthe tubular casing 10, with an allowance for example of 0.01 mm to 0.05mm. The bearings 23, 24 may also be mounted in the tubular casing 10 soas to fit tightly.

The sub-assembly of FIG. 6 may be mounted in the tubular casing 10 (FIG.7 ), for example with a ram, all thereof being pushed with a tubularmounting tool the outside diameter of which is slightly smaller than thediameter of the internal cylindrical surface 21 and the thickness ofwhich is small enough that it interacts only with the external race ofthe second bearing 24. This mounting operation is, therefore, a singlesimple mechanical operation.

The cam ring 11, the corresponding holes 13 of which must be angularlypositioned so that each is placed in front of a hole 12 of the tubularcasing 10, must however be indexed angularly.

In a step E9, the clamping ring 29 and the lip seal 35 are conjointlyplaced in the tubular casing 10, until the lip seal 35 abuts axiallyagainst the second shoulder 22 of the tubular casing 10. The clampingring 29 is then positioned by the lip seal 35.

In a step E10, the wheel hub 4, provided with the studs 5 already inplace, is screwed against the main body 31, thus clamping the clampingring 29, and the dust-proof ring seal 39 is fitted last.

The cam ring 11 is, in the present example, made of 100Cr6 steel, as arethe two bearings 23, 24.

Two races and one ring of the same material (the external race of thefirst bearing 23, the cam ring 11, and the external race of the secondbearing 24) are, therefore, mounted on the internal cylindrical surface21 of the tubular casing 10.

The tubular casing 10, because of its tubular shape and of the materialfrom which it is made, has the mechanical behavior of a tube, inparticular as regards the bending of its walls. In other words, thetubular casing 10 may bulge out if it is locally deformed outwardly bythe cam ring 11, or, in contrast, it may become concave if the cam ring11 is mechanically deformed inwardly.

During its operation, the hydrostatic machine 1 has an increasedlongevity because of the damping, by the tubular casing 10, of thedeformations and shocks exerted on the cam ring 11.

The increased longevity is also due to the presence of a high amount ofcarbon in the bearing steel used for the cam ring 11, this contributingto the very low oxygen content of this steel.

Moreover, as regards the maintenance of the hydrostatic machine 1, thewheel hub 4 can be removed, for example to change or repair the studs 5.Such an operation is here carried out simply by unscrewing the screws 32and by extracting the wheel hub 4 without the interior of thehydrostatic machine 1 being opened, i.e., without any seal needing to beremoved.

The interior of the hydrostatic machine 1 thus remains seal tight,making the operation of removing and installing the wheel hub 4 simpleand clean and of low criticality.

Other variant embodiments of the hydrostatic machine 1 may beimplemented without departing from the scope of the disclosure. Forexample, the materials employed for the cam ring 11 and the tubularcasing 10 may be other materials than those mentioned in the describedexample, provided that the material of the tubular casing 10 has ahigher ductility than the cam ring 11.

In addition, the hydrostatic machine 1 may be fastened to a structureother than that of a vehicle, to a stationary machine for example, andthe rotating drive element may be an element other than a wheel, forexample a gearbox, a machine component or any other transmission deviceor component to be powered.

Alternatively to E470 steel, which was given above by way of an exampleof a ductile material from which the tubular casing 10 could be made,the tubular casing 10 may also be made from a tube of stainless steelthat will advantageously be non-martensitic in order to have asufficient ductility for the job of the tubular casing 10, and in anyevent a higher ductility than the cam ring 11. Preferably, the stainlesssteel will be austenitic, and for example an iron-chromium-nickel alloywith less than 0.1% carbon, such as “18/10” stainless steel. In thiscase, the wheel hub 4 may in addition also be made of stainless steel,this allowing the hydrostatic machine 1 to have an exterior entirely ofstainless steel allowing the hydrostatic machine 1 to be used incorrosive environments such as sea water or corrosive chemicals. In thesame spirit, the tubular casing 10 and the wheel hub 4 may both be madeof another material suitable for a particular application.

What is claimed is:
 1. A hydrostatic machine, comprising: a stator comprising attachments for a structure and comprising a circumferential cam track; a rotor comprising attachments for a rotating drive element and comprising pistons distributed circumferentially and adapted to cooperate with the cam track; and a hydraulic distributor adapted to selectively supply the pistons with hydraulic fluid so that the cooperation of the pistons with the cam track corresponds to a relative rotation of the rotor with respect to the stator, wherein the stator has an inner cylindrical surface of constant diameter and a cam ring comprising, on its inner circumference, the cam track, and that is mounted, by its outer circumference, in the inner cylindrical surface, and wherein two bearings allow rotation of the rotor relative to the stator and are mounted on the inner cylindrical surface of the stator, axially on both sides of the cam ring, the cam ring and the two bearings having the same outer diameter.
 2. The hydrostatic machine of claim 1, wherein the stator further comprises a tubular casing defining the inner cylindrical surface of the stator.
 3. The hydrostatic machine of claim 2, wherein the cam ring exhibits less ductility than the tubular casing.
 4. The hydrostatic machine of claim 3, wherein: the cam ring comprises bearing steel; and the tubular casing comprises non-alloyed steel.
 5. The hydrostatic machine of claim 3, wherein: the cam ring comprises bearing steel; and the tubular casing comprises austenitic stainless steel.
 6. The hydrostatic machine of claim 3, wherein the cam ring and the two bearings comprise a same bearing steel.
 7. The hydrostatic machine of claim 2, wherein the cam ring is fitted tightly in the tubular casing.
 8. The hydrostatic machine of claim 1, further comprising anti-rotation fasteners for coupling the cam ring and the stator.
 9. The hydrostatic machine of claim 1, further comprising: a clamping ring arranged axially against one of the two bearings and against the rotor; and a lip seal between the clamping ring and the stator.
 10. The hydrostatic machine of claim 9, wherein the clamping ring is fastened to the rotor by a hub screwed into the rotor, the hub bearing fasteners for the rotating drive element.
 11. The hydrostatic machine of claim 10, wherein the fasteners borne by the hub for the rotating drive element comprise screws with heads clamped by the hub.
 12. A method for manufacturing a hydrostatic machine, the method comprising: machining a tube of bearing steel to produce a cam ring comprising a cam track; mounting the cam ring and two bearings on a main body of a rotor, the cam ring being clamped between the two bearings to form a sub-assembly; and axially inserting the sub-assembly into a stator comprising an inner cylindrical surface of constant diameter, the two bearings and the cam ring being thereby mounted in the inner cylindrical surface. 